And when you look at studies on kids and cellphones and sleep, the research is even clearer.

"They've gone a step further than the studies on adults," Dr. Bergquist says. "They've shown that just having a cellphone in the bedroom, not turned on, but in the bedroom, affects how sleepy kids are the next day and disrupts their sleep."

So how does your cellphone affect your sleep?

"One possible way is that the blue light from these devices, be they smartphones or computers, this blue light is very stimulating," says Dr. Bergquist. "It makes you mentally alert. It also lowers your level of melatonin. That is the hormone that lets you get drowsy and sleepy at night and it controls your sleep wake cycle."

So how can you get better sleep?

"The most important part is just power down 30 minutes before bedtime," says Bergquist. "Put the phone away."

A survey by the website sleep.org showed more than 70 % of Americans polled say they sleep with their smartphone in their hand or in reach.

But what if you need your phone because it's your alarm clock?

Dr. Bergquist says find another way.

"So, just get the phone out of the bedroom, and use a an alarm clock the good, old-fashioned way," she says.

If you wake up in the middle of the night to use the restroom, as hard as it can be, resist the urge to check your messages or fire off a late night text or email.

"A lot of times getting to that level of alertness can disrupt your ability to fall back asleep," Dr. Bergquist says. "So, stay groggy if you have to get up to use the restroom and just get right back in bed, without checking your text."